Answer: ten-fold
Step-by-step explanation:
The Richter magnitude scale for earthquakes is based on a logarithmic decimal scale (base 10 scale), so that for each increment of a unit on the Richter scale the amplitude of the earthquake wave measured in the seismograph is increased 10 times.
It should be noted that this logarithmic scale is arbitrary and was created by the seismologist Charles Francis Richter, in order to assign a number to quantify the energy released by an earthquake.
Therefore:
The Richter scale is set up so that for every ten-fold increase in the amplitude of the recorded seismic wave, the Richter magnitude increases one number.